There is a constant demand for economical lead-acid batteries with high durability, which serve as starter batteries for automobiles. Such a lead-acid battery includes a battery box including a plurality of cell compartments. Each of the cell compartments includes an electrode plate unit including a positive electrode plate, a negative electrode plate, and a separator. The positive electrode plate is a positive electrode grid filled with paste of lead suboxide powder. The negative electrode plate is a negative electrode grid filled with paste of lead suboxide powder and carbon black. The separator is provided between the positive electrode plate and the negative electrode plate. The abutting electrode plate units are connected in series. The level of an electrolyte poured into the battery box is higher than that of the electrode plate unit. A lid is closed to seal the battery box.
For a long-life lead-acid battery, the negative electrode plate often includes a conductive agent such as carbon black to improve the charge acceptance of the negative electrode plate. In addition, the conductivity depends on the surface area and the amount of the carbon black. The surface area of the CB is often measured with the amount of dibutyl phthalate (DBP) oil absorption.
PATENT DOCUMENTS 1-4 disclose that a negative electrode plate includes carbon black having a large amount of DBP oil absorption (or a large specific surface) to make a long-life lead-acid battery. In particular, PATENT DOCUMENTS 2 and 4 disclose in detail that the CB and a lignin compound are used together to improve the charge acceptance of the negative electrode plate. This CB has a DBP oil absorption of 100-300 ml/100 g or 450-550 ml/100 g. The lignin compound is approximately 0.1-0.6 mass percent relative to a negative electrode active material.